Notch signaling has been implicated in a wide variety of processes from cell fate decisions, tissue patterning and morphogenesis to human disease. In fact, three of the four mammalian Notch genes are associated with cancer while the genes encoding Notch3 and the Notch ligand, Jagged1, are mutated in two inherited human diseases that involve defects in vascular development and maintenance. Furthermore, a role for Jagged1 in angiogenesis is emerging from our studies with mice deficient in Jagged1 that die early during embryogenesis, exhibiting defects in angiogenic remodeling. Interestingly, animals lacking the Notch ligand Delta1 also display a severe hemorrhagic phenotype. Taken together, these data strongly suggest a role for Notch signaling in angiogenesis. To investigate a role for Notch signaling in vascular development we will use the chick embryo chorioallantoic membrane (CAM). Our strategy is to introduce forms of Notch1 or its ligands, Delta1 and Jagged1 into the CAM, using retroviral-mediated expression, that will either activate or inhibit Notch signaling during vascularization of this membrane. Analysis of the effects produced through loss or gain in Notch activity will allow us to determine a role for Notch signaling in angiogenesis. Consistent with a role for Notch signaling during vascularization of the CAM, we have detected the expression of Notch1, Notch2, Jagged1 and Delta1 during development of this membrane and we are currently determining the CAM cell types that express these genes.